West Nile Virus in Horses: Mosquito-Borne Neurological Disease
West Nile virus causes neurological disease in horses. Vaccination is highly effective and essential in endemic areas.
West Nile virus (WNV) is a mosquito-borne flavivirus that causes neurological disease in horses. It was introduced to North America in 1999 and has since spread across the continent. While most infected horses show no signs, approximately 1 in 3 develop clinical disease that can be fatal.
Key Points
- WNV is transmitted by infected mosquitoes, primarily Culex species
- Birds are the reservoir host; horses are dead-end hosts
- Signs include fever, muscle tremors, weakness, ataxia, and seizures
- The case fatality rate is approximately 30-40%
- Vaccination is highly effective and recommended annually before mosquito season
- Mosquito control — eliminating standing water — reduces transmission
How WNV Spreads
WNV is maintained in a bird-mosquito cycle. Infected birds develop high levels of virus in their blood, which infects feeding mosquitoes. Mosquitoes then transmit the virus to horses and humans. Horses are dead-end hosts — they don't develop enough virus to infect mosquitoes. Transmission season coincides with mosquito activity — late spring through autumn in temperate climates.
Prevention
Vaccination is the cornerstone of prevention. Several effective vaccines are available — killed, recombinant, and chimeric. Primary series: 2 doses 3-4 weeks apart, then annual boosters before mosquito season. In endemic areas, some veterinarians recommend 6-month boosters for high-risk horses. Vaccinate foals starting at 4-6 months. Mosquito control — eliminating standing water, using fly sheets and repellents, and stabling during peak mosquito hours (dawn and dusk) — reduces exposure.
When to See a Vet Immediately
- Your horse has fever with neurological signs — stumbling, tremors, or weakness
- Muscle twitching, especially of the face and neck
- Difficulty standing or seizures occur
- Mosquito activity is high and vaccination is overdue
- Recent cases of WNV have been reported in your area
How RVC Can Help
This article is for educational purposes only and does not replace professional veterinary advice. If your pet is showing any symptoms, please contact Royal Veterinary Center immediately at +853 6677 6611.